The exhaust passages of internal combustion engines generally incorporate therein a catalytic converter comprising a three-way catalyst or the like for purifying unburned gases of hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. in the exhaust gases by the catalytic action (oxidation-reduction action). However, when the catalytic converter is at a low temperature such as when the internal combustion engine starts to operate at a low temperature, the catalytic converter is not sufficiently activated. In such a condition, hydrocarbons in particular tend to be emitted in large quantities.
There has been known in the art a hybrid hydrocarbon adsorbing catalyst which comprises a composite combination of a hydrocarbon adsorbent such as zeolite and a three-way catalyst disposed in the exhaust gas passage. The hydrocarbon adsorbing catalyst may comprise, for example, a carrier of honeycomb structure whose surface is coated with a hydrocarbon adsorbent of zeolite and which carries a precious metal such as platinum, palladium, rhodium, or the like as a constituent material of the three-way catalyst. The hydrocarbon adsorbent of zeolite or the like has a function to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100° C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250° C., for example.
In recent years, there have been demands in the art for the recognition of a deteriorated state of an exhaust gas purifier for grasping the need for replacement of the exhaust gas purifier. Such demands also exist for exhaust gas purifiers having hydrocarbon adsorbing catalysts. One conventional technique for evaluating a deteriorated state of a hydrocarbon adsorbing catalyst is known from Japanese laid-open patent publication No. 10-159543, for example. According to the known technique, temperature sensors are disposed respectively upstream and downstream of an exhaust gas purifier having a hydrocarbon adsorbing catalyst. The temperature downstream of the exhaust gas purifier is estimated from the temperature detected by the upstream temperature sensor on the assumption that the hydrocarbon adsorbing catalyst is in a non-deteriorated brand-new state. The actual deteriorated state of the hydrocarbon adsorbing catalyst is evaluated based on the difference between the estimated temperature and the temperature detected by the downstream temperature sensor.
The above technique serves to evaluate the ability of the hydrocarbon adsorbent of the hydrocarbon adsorbing catalyst to adsorb hydrocarbons, but does not evaluate an overall deteriorated state of the exhaust gas purifier including the purifying capability of the three-way catalyst thereof. Therefore, if the three-way catalyst is deteriorated earlier than the ability of the hydrocarbon adsorbing catalyst to adsorb hydrocarbons, then when a deterioration of the hydrocarbon adsorbing catalyst (i.e., a deterioration that demands replacement of the hydrocarbon adsorbing catalyst) is recognized by the technique disclosed in Japanese laid-open patent publication No. 10-159543, the intrinsic ability of the overall hydrocarbon adsorbing catalyst to purify exhaust gases has already been lost. Stated otherwise, the technique disclosed in Japanese laid-open patent publication No. 10-159543 fails to evaluate appropriately the entire evaluated state of the exhaust gas purifier which has the hydrocarbon adsorbing catalyst.
Furthermore, inasmuch as the temperatures detected by the temperature sensors disposed upstream and downstream of the exhaust gas purifier tend to be affected by various factors including an ambient temperature, etc., it is difficult to evaluate, with accuracy, the deteriorated state of the hydrocarbon adsorbent of the hydrocarbon adsorbing catalyst. In addition, the above technique is also disadvantageous as to cost because of the need for the temperature sensors disposed upstream and downstream of the exhaust gas purifier.
Various techniques for evaluating a deteriorated state of a catalytic converter comprising a three-way catalyst or the like are known in the art (e.g., Japanese laid-open patent publication No. 8-144744, Japanese laid-open patent publication No. 2001-182528, and PCT international publication WO/01/46569/A1 filed by the present applicant, etc.). These techniques basically serve to evaluate a deteriorated state of a catalytic converter (a deteriorated state of the ability of a catalyst to purify unburned gases) using output data of an air-fuel sensor such as an oxygen concentration sensor that is disposed downstream of the catalytic converter or air-fuel sensors that are disposed respectively downstream and upstream of the catalytic converter.
The present invention has been made in view of the above background. It is an object of the present invention to provide a deteriorated state evaluating apparatus which is capable of appropriately evaluating a deteriorated state of an overall exhaust gas purifier having a hydrocarbon adsorbing catalyst.